Nickel–molybdenum (Ni–Mo) alloys are promising non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) in alkaline water; however, the kinetic origins of their catalytic activities still remain under debate. In this perspective, we systematically summarize the structural characteristics of Ni–Mo-based electrocatalysts recently reported and find that highly active catalysts generally have alloy–oxide or alloy–hydroxide interface structures. Based on the two-step reaction mechanism under alkaline conditions, water dissociation to form adsorbed hydrogen and combination of adsorbed hydrogen into molecular hydrogen, we discuss in detail the relationship between the two types of interface structures obtained by different synthesis methods and their HER performance in Ni–Mo based catalysts. For the alloy–oxide interfaces, the Ni4Mo/MoOx composites produced by electrodeposition or hydrothermal combined with thermal reduction exhibit activities close to that of platinum. For only the alloy or oxide, their activities are significantly lower than that of composite structures, indicating the synergistic catalytic effect of binary components. For the alloy–hydroxide interfaces, the activity of the NixMoy alloy with different Ni/Mo ratios is greatly improved by constructing heterostructures with hydroxides such as Ni(OH)2 or Co(OH)2. In particular, pure alloys obtained by metallurgy must be activated to produce a layer of mixed Ni(OH)2 and MoOx on the surface to achieve high activity. Therefore, the activity of Ni–Mo catalysts probably originates from the interfaces of alloy–oxide or alloy–hydroxide, in which the oxide or hydroxide promotes water dissociation and the alloy accelerates hydrogen combination. These new understandings will provide valuable guidance for the further exploration of advanced HER electrocatalysts.